iio: adc: xilinx: Remove platform_get_irq from xadc_remove function

This patch avoids getting irq number in xadc_remove function. Instead
store 'irq' in xadc struct and use xadc->irq wherever needed.
This patch also resolves a warning reported by coverity where it asks to
check return value of platform_get_irq() for any errors in xadc_remove.

Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>